#67.Add Binary [LeetCode Grind 75 in Java]

class Solution {
    public String addBinary(String a, String b) {
        int ptra = a.length() - 1;
        int ptrb = b.length() - 1;
        StringBuilder ans = new StringBuilder();

        int carry = 0;
        while(ptra >= 0 || ptrb >= 0){
            int sum = carry;
            if(ptra >= 0) sum += a.charAt(ptra --) - '0';
            if(ptrb >= 0) sum += b.charAt(ptrb --) - '0';

            carry = (sum > 1) ? 1 : 0;
            ans.append(sum % 2); 
        }

        if(carry != 0) ans.append(carry);
        return ans.reverse().toString();
    }
}